1. Field of the Invention
This invention relates to electronic devices for the measurement of body fat, and more particularly to those devices which measure body fat by applying a low-level constant RMS value alternating current to the body.
2. Description of the Prior Art
Measurement of body composition is assuming greater importance in assessing nutritional status in both health and disease. Direct measurement of body composition has been limited to research centers using hydrostatic weighing or isotope-dilution techniques. An indirect measurement technique has been developed based upon a determination of body electrical impedance.
The method for determining body impedance is based upon the nature of the conduction of an applied electrical current in the human body. In biological structures, application of a constant, low-level alternating current produces an impedance to the spread of the current that is frequency dependent. The human body contains intra- and extracellular fluids that act as electrical conductors and cell membranes that act as imperfect reactive elements. At low frequencies (around 1 kHz), the current mainly passes through the extracellular fluids, while at higher frequencies (500-800 kHz) it penetrates the intra- and extracellular fluids. Thus, body fluids and electrolytes are responsible for electrical conductance (the inverse of resistance) and cell membranes are involved in capacitance.
The use of bioelectrical impedance measurements to determine fat-free mass is based upon the principle that the impedance of a geometrical system is related to conductor length and configuration, the impedance to the flow of current can be related to the flow of current as ##EQU1## where Z is impedance in ohms, p is volume resistivity in ohm-cm, L is conductor length in cm, and A is conductor cross-sectional area in cm.sup.2. Multiplying both sides of the equation by L/L gives: ##EQU2## where AL equals the volume V. Rearranging this question yields, ##EQU3##
In the human body, electrical conduction is related to the water and electrolyte distribution in ,.the biological conductor. Because fat-free mass contains virtually all the water and conducting electrolytes in the body, conductivity is far greater in the fat-free mass that in the fat mass of the body. The electrically determined biological volume V is inversely related to Z, and thus it is also inversely rated to resistance R and reactance Xc, since ##EQU4## Because the magnitude of reactance is small relative to resistance, and resistance is a better predictor of impedance than is reactance, volume can be expressed as ##EQU5## where L is standing height in cm and R is resistance in ohms. Although there are difficulties in applying this general principle in a system with as complex geometry and bioelectrical characteristics as the human body, this relationship has been used to derive models for the prediction of human body composition by assuming that the body is a series of connected cylinders.
Determinations of resistance and reactance have been made using four terminal impedance plethysmographs. Examples of such plethysmographs include the model 101, manufactured by RJL Systems of Detroit, Mich. The four terminal method has been used to minimize contact impedance or skin-electrode interactions. As a general procedure, measurements were made about two hours after eating and within 30 minutes of voiding. The patient, clothed but without shoes or socks, lied supine on a cot. Aluminum foil spot electrodes were positioned in the middle of the dorsal surfaces of the hands and feet proximal to the metacarpal-phalangeal and metatarsal-phalangeal joints, respectively, and also medially between the distal prominences of the radius and the ulna and between the medial and lateral malleoli at the ankle. A thin layer of electrolyte gel was applied to each electrode before application to the skin. An excitation current of 800 .mu.A at 50 kHz was introduced into the patient at the distal electrodes of the hand and foot and the voltage drop was detected by the proximal electrodes.
According to Ohm's Law the electrical impedance Z to alternating current of a circuit was measured in terms of voltage E and current I as ##EQU6## EQU Z=E/I.
By using phase sensitive electronics, one could quantify the geometrical components of Z. Resistance R is the sum of in-phase vectors, and reactance Xc is the sum of out-of-phase vectors. A phase discriminator was used to measure the phase angle to produce resistance and reactance measurements.
This technique provided a deep homogenous electrical field in the variable conductor of the body. Determinations of resistance and reactance were made using electrodes placed on the ipsilateral and contralateral sides of the body. The lowest resistance value for an individual was used to calculate conductance (h.sup.2 /R) and to predict fat-free mass. The accuracy of this method has been found to be within 2%. Statistical models have been developed to estimate total body water and fat-free mass in adults.
To provide an accurate measure of resistance and reactance, it is important to provide a constant current source. Existing plethysmoqraphs have been able to provide a constant current source only by using expensive circuitry. Plethysmographs using less expensive circuitry have been unable to supply a constant current source with the associated accuracy to provide accurate resistance and reactance measurements.
In addition, it is apparent that the measurement device must be properly connected to the body of the patient. If any of the four electrodes is improperly located on the patient's body or is improperly connected to the measurement device, the resulting measurements will be erroneous.